The invention and the application of the laser are considered to be the most important achievement of science in the twentieth century. Today, the goods related to the laser technology around the modern life have had too many things for the eye to see for a long time. The high-energy laser, e.g. the pulse laser, is widely used in the cutting of the industry or the surgical operation, which sufficiently affirms the value of the laser technology.
A fiber ring cavity is a conventional device used for producing an output laser pulse PW1. Please refer to FIG. 1, which is a schematic diagram showing a conventional configuration of a fiber ring cavity. As shown, a ring cavity 10 includes a wavelength division multiplexing device 11, a fiber 12, an optical gain fiber 13, a polarization controller 14 and an optical coupler 15. The wavelength division multiplexing device 11 is coupled to the fiber 12, and couples a pump light beam PB1 into the ring cavity 10. The optical gain fiber 13 includes an optical gain dope, e.g. a rare earth element substance, a transition metal element substance or a nonlinear effect substance. The pump light beam PB1 goes into the optical gain fiber 13 along the clockwise direction and excites ground-state electrons of the optical gain dope in the optical gain fiber 13 to positions of the excited state, so that photons having different wavelengths are released. These photons travel along the fiber 12 within the ring cavity 10 to form light beams having different wavelengths.
With the progress of time, the pump light beam PB1 is continuously coupled into the ring cavity 10 through the wavelength division multiplexing device 11 and causes the optical gain fiber 13 continuously releases light beams having different wavelengths. However, only some light beams can develop into the standing-wave state and continuously exist in the ring cavity 10, and the other light beams gradually decay to finally disappear, wherein an integer multiple of the half wavelength of each of the some light beams is just equal to the total length of the ring cavity 10. As the arrows, marked in the inner side of the ring cavity 10, show in FIG. 1, a light beam goes forward along the counterclockwise direction to form a standing wave. The polarization controller 14 adjusts the polarization of the light beam passing through the polarization controller 14 and causes the light beam to form laser light beams, which are in a co-polarization state. The laser light beams interfere mutually so as to form a laser pulse PC1 output by the polarization controller 14, i.e. the polarization controller 14 is used for mode-locking the laser pulse PC1. The ring cavity 10 causes the laser pulse PC1 to propagate within the ring cavity 10 for providing a laser pulse PD1, a laser pulse PU1 and a laser pulse PH1 to the optical gain fiber 13, the optical coupler 15 and the polarization controller PH1 respectively.
The laser pulse PD1 propagates into the optical gain fiber 13 to further induce photons out so as to make an optical gain, which causes the intensity of the laser pulse PD1 to continuously increase for forming an amplified laser pulse, wherein the photons are homologous with the intracavity laser pulse PC1. In the meantime, the amplified laser pulse propagates along the counterclockwise direction in the ring cavity 10. When the laser pulse PH1 passes through the optical coupler 15, a small portion 10-20% of the laser pulse PH1 is output by the optical coupler 15 to be the output laser pulse PW1 as shown in FIG. 1.
It is often necessary for the practical application of the laser pulse to handle two important parameters, the wavelength and the time pulse width. To adjust the wavelength of the laser pulse is necessary for fitting the processed material property. It is also necessary for different applications to be able to control the time pulse width of the laser pulse. Although it seems that the abovementioned configuration of the ring cavity 10 is simple, it is difficult to effectively control the wavelength and the time pulse width, so that further improvement is required.